1. Field of the Invention
This invention relates to a gas-solid contact method and apparatus and, more specifically, this invention relates to a method and apparatus for flue gas desulfurization and cleaning.
2. Description of the Prior Art
Concern surrounding the level of gaseous and particulate pollutants emitted from industrial and utility installations, such as in flue gas emitted from coal- and oil-fired industrial and utility boilers, has increased dramatically in recent years. Effective means for removing pollutants are required in order to make practical widespread use of relatively economical high sulfur- and ash-content coal, known to exist in great quantities in certain areas of the United States. Such coal is known to have relatively high heating values, expressed in BTU of available heat per pound.
The use of wet scrubbers for flue gas desulfurization and removal of particulates is well known. In general, prior scrubbers effected intimate contact of flue gas with aqueous or other solutions of lime, soda ash or other materials suitable for absorption of SO.sub.2, NO.sub.x and particulate pollutants from the gas. Gas-liquid scrubbing contact was typically effected by means of sprays, packed beds, or other means well known in the art for effecting gas-liquid contact.
Wet scrubbers exhibit numerous disadvantages when used in flue gas desulfurization processes. Reactant or absorbent slurries used in such scrubbers typically contain no more than 25 weight percent solids, and thus require pumping systems of substantial capacity for supply, removal and recirculation of slurry. Further, many wet systems exhibit a substantial pressure drop, thus adding to the required pumping and blower capacity.
Even in scrubbing systems utilizing an alkali absorption medium, careful pH control is required to prevent corrosion resulting from excessive neutralization of reactant or absorbent and the consequent formation of acidic solutions. To prevent corrosion, wet scrubbing apparatus are often fabricated of relatively expensive stainless steel or other corrosion-resistant alloys.
Scaling, plugging and fouling is a common problem in alkali slurry and other wet flue gas scrubbers, as is mist carryover and the requirement of mist elimination equipment. Discharge of wet gas to the discharge stack, as a result of incomplete mist removal, causes additional corrosion problems. Careful control of the temperature of entering flue gas is required to avoid vaporization of scrubbing liquid and consequent steam production, which may result in undesirable pressure increases within process equipment. Contact of flue gas with scrubbing liquid results in waste of heat from the gas, which could otherwise be recovered. Resulting relatively low outlet temperatures from the scrubbing systems as well as mist carryover, cause stack and system corrosion.
Spent reactant or absorbent must be removed from slurry by filtration or other means. Drying of the resulting filter cake requires water removal equipment.